Past industrial activities have contaminated sediments in many streams, rivers, lakes, and harbors. The contaminated sediment requires remediation to mitigate its potential impact on ecological receptors, human health, or environmental media. An overview of sediment remediation options is provided below.
In-situ Capping—In-situ capping isolates contaminated sediment from the surrounding surface water body or ecological receptors by placing a protective cover over the contaminated sediment area.
In-Situ Treatment—In-situ treatment refers to treatment of the contaminated sediment at its current location without removal. The treatment methods include biological, chemical, and physical processes.
Removal—Removal is a necessary step for other remediation methods such as ex-situ treatment, off-site disposal or on-site disposal. The most common removal method is dredging. Excavation is also used if the sediment is under a shallow water body that may be drained temporarily using a simple and economical surface water barrier.
Ex-situ Treatment—In this approach, contaminated sediment is removed from its current location and treated. Ex-situ treatment methods include bioremediation, chemical treatment, soil washing, solidification/stabilization and others.
Off-site Disposal—Even after ex-situ treatment, the quality of treated sediment may not fully meet all regulatory requirements. In this case, the treated sediment is taken to an off-site disposal facility (sanitary, industrial or hazardous waste landfill) for safe disposal.
On-site Disposal—Contaminated sediment may be removed and contained, with or without treatment, in an engineered disposal facility built at the project site solely for disposal of the target sediment. The disposal facility filled with sediment is closed as a landfill. Therefore, sediment dewatering is essential. Two common dewatering methods are mechanical dewatering and geotube dewatering.
In mechanical dewatering, dredged sediment is pumped to a mechanical dewatering unit (e.g., a centrifuge, a belt press, or a filter press), dewatered, and cake is placed in the disposal facility. Often, the cake requires solidification/stabilization as cake from mechanical dewatering cannot support earthwork equipment used for disposal work.
Geotube dewatering uses geotubes for dewatering. Geotubes are large filter bags made of geotextile. Dredged sediment is pumped into a geotube and water is allowed to drain, leaving solids in the geotube. After the geotube is filled with pumped-in dredged sediment, the sediment is allowed to drain for some time. When the geotube collapses as water is drained, more dredged sediment is pumped into the geotube. After cycles of filling and draining, the geotube is filled with “drained” sediment. The drained sediment may be dewatered further, if desired, by evaporative drying for several weeks. The dewatered sediment may be taken off site for disposal. For on-site disposal, geotubes may be deployed within the disposal pond before they are filled.
Contained Aquatic Disposal—Contained aquatic disposal is underwater disposal and capping of dredged sediment in natural depressions, excavated pits or bermed areas at the bottom of water bodies. This method is often used for disposal of the sediment dredged from harbors and urban waterways where on-site disposal is not feasible due to limited land area. The disposal sites are selected from areas with a sufficient water depth (to avoid interruption of navigation) and low water energy (to avoid erosional loss of contained sediment).
Consolidation refers to a process of soft clayey soils subject to a load undergoing volume reduction and strength gain as a result of water being squeezed out of the loaded soil volume. As clayey soils do not allow water to flow out easily due to its very low hydraulic conductivity, drainage pathways are provided in the soil volume to accelerate consolidation. The most common way of providing drainage pathways is to insert wick drains vertically into the clay layer with a typical spacing of about 1.5 m. A wick drain is a long strip about 0.5 cm thick and 10 cm wide and consists of a plastic core wrapped with geotextile filter. Wick drains facilitate flow of water from soft clayey soils to the ground surface.
Accelerated consolidation with wick drains has been used for numerous construction projects on soft clayey soils. However, it has not been used for dewatering of dredged sediment in environmental remediation due to one critical limitation. As consolidation is a method of stabilizing a full layer of soft soil, it is applicable to dredged sediment after the disposal operation is completed. However, consolidation dewatering after filling a disposal pond with dredged sediment is not practical for the reasons described below.
To illustrate the point, suppose that consolidation dewatering is attempted for disposal of dredged sediment. Dredged sediment typically contains less than 10% solids by weight as it is pumped as a slurry. After settling in the disposal pond, its typical solids content is around 35% by weight, equivalent to 17% solids and 83% water by volume. As this is too soft to place a final cover for closure, the dredged sediment requires dewatering, in this case by consolidation. The pond surface has to be stabilized first by draining and natural drying to allow equipment access. This step takes very long. The subsequent steps of consolidation work include covering the surface with a geotextile, spreading 0.5 to 1.0 m of sand (top blanket drain) over the geotextile, installing vertical wick drains into the soft sediment with an installation rig working on the top blanket drain, and loading with thick earth fill. As this fill cannot be placed in one step on the very soft sediment, it has to be placed in small lifts, allowing time for consolidation and strength gain before applying the next lift. Thus, this loading step also takes a long time. A large settlement, typically about 50% to 70% of the initial thickness, occurs as a result of consolidation. The final step of pond closure would be surface grading and final cover installation. Surface grading requires the entire fill, or 50% to 70% of the thickness of the consolidated sediment in the disposal pond, to remain in the pond.
The steps described above signify three major problems in consolidation dewatering for on-site disposal of dredged sediment. First, these steps take too long, particularly in stabilizing the surface for equipment access and in applying the load in several lifts. Second, the capacity of the disposal pond is wasted by fill equivalent to 50 to 70% of the pond capacity. Third, the above two reasons make consolidation dewatering costly and impractical. For these reasons, consolidation dewatering is not viable for disposal of dredged sediment in environmental remediation, unless technical improvements are made. The above problems can be overcome if the sediment in the disposal pond is consolidated while dredged sediment is being discharged into the pond. Thus, it would be desirable to devise a method of consolidation dewatering concurrent with discharge of the dredged sediment into the disposal pond.
In achieving the goal stated above, vacuum loading will play a key role. Vacuum has been often utilized as a means of loading for consolidation projects. In this method, the ground surface is covered with an impermeable membrane and vacuum is applied to the underside of the membrane. This creates an effect of atmospheric pressure working as a load. Although vacuum consolidation offers some advantages, it is often troublesome due to incomplete seals along the edge of the membrane and its cost is significant. A Dutch firm COFRA (see COFRA webpage) practices a vacuum loading method that does not require membrane sealing by connecting the top of vertical wick drains with sealed vacuum lines within the soft clay layer, which is almost impermeable. The present invention intends to extend vacuum consolidation application to horizontal drains using self-sealing properties of fluid earthen medium which is the target for consolidation.
The Corps of Engineers performed a research project evaluating ways of stabilizing dredge spoils from navigation dredging and demonstrated that vacuum underdrainage is an effective way of stabilizing dredge spoil (Hammer, 1981). In this method, a layer of bottom blanket drain is installed in the disposal facility, dredge spoil is discharged, and a vacuum is applied to the bottom blanket drain.